I've looked a little for specific links proving whether memblur is a complete reset in a mobs hp or not and found nothing in that regard (because it wasn't that way at all ever as far as I can recollect). Lots of discussion on the use of it in various situations where a complete hp reset would have been a quick raid wipe (
changing rampage tank in Velious). The only use for griefing was in order to win a "dps race" when one raid was trying to KS another (
memblur AoW when the mob was already damaged and then disc it to death) in Velious.
Found no reference in posted patch notes archive.